Process and device for producing high-purity liquid chemicals

ABSTRACT

For a process for producing high-purity liquids, particularly liquid chemicals, by distillation it is proposed that the liquid to be purified (1) is heated by microwave radiation (5) preferably in the uppermost layers.  
     To implement this process a device with a liquid container (3), with a heat source (5) acting on the liquid container and with a condensation device (8, 9) connected to the liquid container is proposed, which is distinguished in that the heat source (5) in the form of a microwave radiation source is arranged above the liquid to be purified (1) in such a way that preferably the uppermost layers of the liquid (1) are heated.

[0001] The invention relates to a process for producing high-purity liquid chemicals, by distillation wherein the liquid to be purified is heated by microwave radiation, and a device for implementing this process having a heat source acting on the container of liquid and a condensation device connected to the liquid container wherein the heat source is a microwave radiation source that is arranged above the liquid to be purified.

[0002] Chemicals usually contain a certain amount of impurities. To separate the liquid chemical from the impurities it is usually distilled. Distillation comprises evaporating the liquid and condensing the vapors to produce the distillate which is collected together (simple distillation) or in a separate manner according to boiling ranges (fractional distillation). As simple distillation does not achieve complete mixture separation it is only used when high purity is unimportant. Fractional distillation is used, for example, in the treatment of petroleum or the production of alcohol.

[0003] Since, according to Raoult's law, the higher-boiling substance also sends a quantity corresponding to its content and vapor pressure into the vapor of the lower-boiling substance, accurate separation is only possible by means of a multiplicity of successive distillation steps. Rectification, in which a so-called distillation column is connected between evaporator and cooler, is a process for the very fine separation of liquid mixtures.

[0004] The object of the invention is to provide a further distillation process for producing high-purity liquid chemicals and a device for implementing this process.

[0005] The process according to the invention comprises heating the liquid to be evaporated by means of microwave radiation preferably in the uppermost layers while deeper layers remain cooler. This measure may prevent higher-boiling impurities from being entrained into the vapor by bubble formation. In the corresponding device the heat source in the form of a microwave radiation source is arranged above the liquid to be purified.

[0006] In development of the invention, convection within the liquid is extensively prevented by introducing horizontal nets or perforated plates into the liquid. This measure may also reduce the effect by which impurities are entrained upwards by the heat transfer within the liquid and find their way into the distillate.

[0007] Furthermore, the liquid level in the liquid container is kept constant during the distillation process in that the liquid container is supplied from a level system. For this purpose the liquid container is connected to a storage vessel from which a pump pumps the chemical to be purified into an overflow pipe which is connected in the upper part to the storage vessel and whose bottom end opens into the liquid container at the bottom.

[0008] All elements which come into contact with the chemical in the course of the distillation process must be resistant to the chemical used. Glass may be used as material in the case of chemicals which are not excessively aggressive; if the chemical to be purified is hydrofluoric acid, for example, which attacks glass, polytetrafluoroethylene (PTFE) may be used for example.

[0009] The sub-claims provide further embodiments and developments of the invention.

[0010] An embodiment of the invention will now be explained with the aid of the drawing, in which:

[0011]FIG. 1 shows a schematic view of an embodiment of the device according to the invention, which will also serve to explain the process according to the invention;

[0012]FIG. 2 shows a further embodiment of a device according to the invention in a simplified view; and

[0013]FIG. 3 shows details of the device of FIG. 2 in two different, simplified views.

[0014] The process according to the invention and an embodiment of a device according to the invention will be explained in principle below with the aid of FIG. 1. The liquid chemical to be purified 1 is conveyed via an inlet 2 to the lower region surface 4 constant during the distillation process, the liquid container 3 is supplied from a level system via the inlet 2.

[0015] The level system is shown on the right-hand side of FIG. 1. The liquid chemical to be purified 1 is in a storage vessel 12. From there it is pumped by means of a pump 13 into an overflow pipe 14 which is connected in the upper part to the storage vessel 12 and whose bottom end opens via the inlet 2 into the lower region of the liquid container 3. By means of the level system formed in this way, the level of the liquid surface 4 is constantly kept at the level of the upper end of the overflow pipe 14.

[0016] In the embodiment explained here, the liquid container 3 is located in a microwave sample chamber 15 and must consist of a microwave-permeable material. The microwave radiation 5 is generated by a microwave generator 25 and introduced in the upper region of the microwave sample chamber 15 via one or more introduction openings 16. Generally conventional elements may be used to generate the microwave radiation 5 and transfer it from the microwave generator 25 to the introduction openings 16. As the microwave radiation 5 comes from above and as the liquid 1 absorbs the microwave radiation 5, the uppermost layers of the liquid 1 are greatly heated whereas the lower region remains cooler.

[0017] Microwave radiation is clearly superior to other heat sources, such as, for example, infra-red radiation. Compared with infrared, the use of microwave beams 5 has the advantage that indirect heating of the upper layers of the liquid 1 is possible if a microwave-absorbing material is introduced in the region of the uppermost layers of the liquid 1 just below the surface 4, as will be explained in greater detail below.

[0018] At least two temperature measurement devices 6 a and 6 b, which are preferably formed as infrared thermosensors, are used to measure the temperature distribution of the liquid 1 in the liquid container 3. A first IR thermosensor 6 a measures the temperature in the hotter upper liquid layers and a second IR thermosensor 6 b measures the temperature in the lower cooler region of the liquid 1. The thermosensors 6 a, 6 b are connected to a control unit 24 which is connected in turn to the pump 13 of the level system and to the radiation source 25.

[0019] According to the measured result of the temperature distribution, the supply of cold liquid to be purified 1 into the liquid container is controlled on the one hand and the output of the radiation 5 incident on the surface 4 of the liquid 1 on the other hand. By keeping the temperature constant in the upper layers of the liquid 1 and in the lower region, it is possible to prevent any extensive intermixing.

[0020] A condensation device is arranged above the liquid surface 4. It is still inside the sample chamber 15 and comprises a guide pipe 8 running downwards in an inclined manner, in which a multiplicity of vapor flow holes 23 are arranged, through which the rising vapor 7 can penetrate into the guide pipe 8. A cooling finger 9 extends concentrically in the guide pipe 8, in which finger a coolant supply pipe 17 runs, also concentrically, and there is a spacing between this pipe and the cooling finger 9. The coolant flows internally in through the coolant supply pipe 17 and flows out again at the other open end of the coolant supply pipe, so that if flows back in the intermediate chamber between coolant supply pipe 17 and cooling finger 9 and finally leaves the condensation device again via a coolant outlet 18. Water or silicone oil, which scarcely absorbs the microwaves and is not directly heated, is suitable as coolant for example.

[0021] With correspondingly controlled output of the radiation 5 onto the surface 4 of the liquid 1 the uppermost layers are heated so highly that the lowest-boiling component evaporates out of the liquid 1. The vapor 7 escaping upwards penetrates through the vapor flow holes 23 of the guide pipe 8 and condenses on the cooling finger 9. The condensate 10 drips down onto the inside of the guide pipe 8 and flows along the guide pipe arranged in an inclined manner, into a collecting vessel 11.

[0022] All elements which come into contact with the chemical in the course of the distillation process, such as liquid container 3, condensation device 8, 9 or level system 12, 13, 14 must be resistant to the chemical used. Glass may be used as material in the case of chemicals which are not excessively aggressive; if the chemical to be purified is hydrofluoric acid, for example, which attacks glass, polytetrafluoroethylene (PTFE) may be used for example.

[0023] As a further structural feature of the device, a net and/or a perforated plate 20 of microwave-absorbing material is horizontally introduced just below the surface level 4 in the liquid container 3. This plate 20 permits indirect heating of the uppermost layers of the liquid 1 with microwave radiation 5. This measure means that the heating of the liquid 1 is concentrated more strongly on the uppermost layers in the liquid container 3 so that entrainment of bubbles of higher-boiling impurities in the liquid 1 is restricted still further and a higher purity of the liquid chemical may be achieved.

[0024] Further nets or perforated plates 21 are introduced horizontally in the liquid container 3 underneath the microwave-absorbing perforated plate 20. These plates 21 should preferably be microwave-permeable, particularly when they are arranged in deeper layers in the liquid 1, in order to prevent indirect heating of these deeper layers. This structural measure permits a reduction of convection, i.e., of the heat transfer within the liquid 1. With convection—if it takes place from the bottom upwards—impurities may again be entrained and limit the purity of the liquid chemical which may be achieved with the process.

[0025] A higher degree of purity can be achieved by means of the measures according to the invention than with processes known hitherto, as only the uppermost liquid layer is greatly heated and thus scarcely any higher-boiling impurities are entrained into the vapor 7 by means of bubble formation. Purity is also improved by the reduction of the convection within the liquid 1.

[0026] The process described above is suitable for producing high-purity liquids of any kind, particularly for liquid chemicals such as are required in the production of semi-conductors. An example of such a chemical is hydrofluoric acid.

[0027]FIG. 2 shows a further embodiment of a device according to the invention in simplified representation. Parts also shown in FIG. 1 are denoted by the same reference numerals in FIG. 2; their function is not explained again. Level system, thermosensors, control unit and microwave generator are also present in this embodiment even though they are not illustrated for reasons of simplicity; the plates and/or nets for surface heating and for convection reduction may, of course, also be used in this case.

[0028] In this embodiment the liquid container 3 is tubular in design and is arranged in the microwave sample chamber 15 in an inclined manner and not horizontally. A drain device 22 is also arranged in the lower region of the liquid container 3 so that the contents of the liquid container 3 may be emptied in a simple manner, for which the inclined position of the liquid container 3 is particularly advantageous. The drain device 22 also permits a draining of the liquid 1 during the distillation process so that an excessively high impurities content in the liquid container 3 may be avoided.

[0029] In this case, the entire condensation device comprising guide pipe 8 and cooling finger 9 arranged therein is arranged outside the microwave sample chamber 15. The rising vapor 7 finds its way through a pipe 26 in the upper region of the liquid container 3 above the liquid surface 4 to the guide pipe 8 and enters the guide pipe 8 again through vapor flow holes 23 in order to condense on the cooled cooling finger 9 and flow off into a collecting vessel 11.

[0030] To prevent the microwave radiation 5 from being able to penetrate downwards outside the liquid container 3 and thence into the lower layers of the liquid 1 in the liquid container 3, so-called lamellar traps 19 are arranged between the internal wall of the microwave sample chamber 15 and the external wall of the liquid container 3. The lamellar traps consist of a microwave damping or absorbing material and preferably have a mutual spacing, which corresponds to approximately one-quarter of the wavelength of the incident microwave radiation 5, in order thereby further to intensify the absorption of the microwave radiation 5.

[0031] Finally, FIG. 3 shows the embodiment of FIG. 2 in two different views, again greatly simplified, in order to explain the system of the microwave introduction and the lamellar traps 19 in greater detail. FIG. 3b shows the same view as FIG. 2, and FIG. 3a shows the device of FIG. 2 and/or FIG. 3b from the right.

[0032] The microwaves 5 generated by the microwave generator 25 are introduced into the microwave sample chamber 15 from above and heat the liquid in the microwave-transparent liquid container 3. The lamellar traps 19 arranged round the liquid container in the same direction as the latter and running parallel to each other absorb the microwave radiation 5 incident from above, by means of their microwave-absorbing properties on the one hand and because of their mutual spacing of a quarter of the wavelength of the microwaves 5 on the other hand. Because of this design the microwaves 5 do not penetrate into the lower region of the sample chamber 15 and cannot therefore heat it. 

I claim:
 1. A process for producing high-purity liquids, particularly liquid chemicals, by distillation, wherein the liquid to be purified is heated by microwave radiation preferably in the uppermost layers.
 2. A process as claimed in claim 1 , wherein the liquid is indirectly heated by the microwave radiation.
 3. A process as claimed in claim 1 , wherein the level of the liquid surface is kept constant during the process.
 4. A process as claimed in claim 1 , wherein the vapor arising through heating of the uppermost layers of the liquid condenses above the liquid surface on a condensation device and the condensate flows into a collecting vessel.
 5. A process as claimed in claim 1 , wherein the temperature distribution in the liquid is monitored by at least two temperature measurement devices, at least one temperature measurement device of which measures the temperature in the uppermost layer of the liquid and at least one temperature measurement device measures the temperature in the lower region of the liquid, to control the output of the radiation incident on the liquid surface and the supply of cold liquid to be purified into the liquid container.
 6. A device for implementing the process as claimed in one of claims 1 to 5 , with a liquid container, with a heat source acting on the liquid container and with a condensation device connected to the liquid container, wherein the heat source is a microwave radiation source and is arranged above the liquid to be purified, in such a way that preferably the uppermost layers of the liquid are heated.
 7. A device as claimed in claim 6 , wherein the liquid container is arranged in a sample chamber in which the microwave radiation generated by a microwave generator is introduced via introduction openings.
 8. A device as claimed in claim 6 , wherein the liquid container is connected to a level system for keeping constant the level of the liquid surface in the liquid container.
 9. A device as claimed in claim 8 , wherein the level system comprises a storage vessel for the liquid to be purified, a pump and an overflow pipe into which the liquid is pumped by means of the pump from the storage vessel, in which the overflow pipe is connected in the upper part to the storage vessel, in which the overflow pipe is connected in the upper part to the storage vessel and the bottom end of the overflow pipe opens into the lower region of the liquid container.
 10. A device as claimed in claim 6 , wherein the condensation device is arranged above the liquid surface.
 11. A device as claimed in claim 6 , wherein the condensation device is arranged inside or outside the microwave sample chamber.
 12. A device as claimed in claim 6 , wherein the condensation device comprises a guide pipe running downwards in an inclined manner, with vapor flow holes, and a cooling finger extending concentrically in the guide pipe.
 13. A device as claimed in claim 12 , wherein a coolant supply pipe, which has a spacing from the cooling finger and is open at the bottom end, runs concentrically in the cooling finger so that a coolant flows in internally through the coolant supply pipe, flows out again at the other open end and flows back in the intermediate chamber between coolant supply pipe and cooling finger, in order finally to leave the condensation device via a coolant outlet, or vice-versa.
 14. A device as claimed in claim 13 , wherein water or silicone oil is used as coolant for the cooling finger.
 15. A device as claimed in claim 11 , wherein a condensate flowing off in the guide pipe is collected in a collecting vessel.
 16. A device as claimed in claim 6 , wherein at least two temperature measurement devices are present for monitoring the temperature distribution in the liquid container, of which at least one temperature measurement device measures the temperature in the uppermost layer of the liquid and at least one temperature measurement device measures the temperature in the lower region of the liquid, wherein a control unit is provided to control the output of the radiation incident on the liquid surface and/or the supply of cold liquid to be purified into the liquid container, and wherein the temperature measurement devices are connected to the control unit.
 17. A device as claimed in claim 16 , wherein the temperature measurement devices are infrared thermosensors.
 18. A device as claimed in claim 6 , wherein a net or a perforated plate of microwave-absorbing material is introduced approximately horizontally in the liquid container just below the liquid surface, in order indirectly to heat the uppermost layers of the liquid by means of microwave radiation.
 19. A device as claimed in claim 6 , wherein nets or perforated plates arranged horizontally or in an inclined manner are introduced in the liquid container in order to reduce convection within the liquid.
 20. A device as claimed in claim 19 , wherein the nets or perforated plates consist of a microwave-permeable material.
 21. A device as claimed in claim 6 , wherein the liquid container is surrounded by lamellar traps of microwave damping or absorbing material.
 22. A device as claimed in claim 21 , wherein the lamellar traps arranged approximately parallel to each other have a spacing which corresponds approximately to one quarter of the wavelength of the incident radiation.
 23. A device as claimed in claim 6 , wherein all elements of the device which come into contact with the liquid in the course of the distillation process consist of a material resistant to the liquid used. 